Many industries use non-destructive inspection systems to inspect objects. These inspection systems typically generate signals indicative of physical characteristics of the objects. However, there is often noise and/or distortion, collectively referred herein to as distortion, in the signals. The distortion can limit the usefulness of the signals, and thereby hinder the inspection.
For example, many large structural castings (e.g., diffuser cases, liners, etc.) are used in the aerospace and the power generation industries. These castings typically comprise a material having a crystalline microstructure characterized by a plurality of individual crystals, commonly referred to as grains. The grains are often relatively large in size. For example, IN718, a high strength nickel alloy containing Chromium, Cobalt, Titanium, and Aluminum, has grains of sizes in the range of from about 0.060 inches to about 0.180 inches. The grains are generally randomly oriented throughout the casting.
The castings sometimes contain internal defects such as microshrinkage, cracks, and inclusions. These defects are all characterized by a relatively low density and are accordingly referred to as low-density defects. Some of these defects may be subtle in that they are relatively small in size and/or may have only slightly different density than that of regions surrounding the defect. For example, a microshrinkage defect is a region characterized by clusters of slight porosity in the microstructure resulting in slightly lower density than that of surrounding regions. However, even a subtle defect can be significant enough to effect the reliability of the casting.
Radiographic inspection systems have typically been used to try to detect internal defects in the castings. Radiographic inspection systems pass x-rays through an object to produce x-ray photographs (i.e., x-ray images) indicative of physical characteristics of the object.
However, the relatively large, generally randomly oriented grains in the microstructure cause a significant amount of unwanted scattering and diffraction of the x-rays as they pass through the casting. Consequently, the x-ray images contain a high level of distortion. The distortion is in the form of intensity variation (sometimes referred to as mottling). The intensity variation associated with the distortion is often greater in magnitude than intensity variation associated with some subtle but significant defects. Hence, the distortion makes it difficult or impossible to determine that there are no subtle but significant defects by means of conventional inspection of the x-ray images. For instance, it has traditionally not been possible to detect a 0.100 inch long microshrinkage defect in a nickel casting having a wall thickness of 0.250 inches on the basis of a conventional x-ray image.
Due to such distortion, some subtle but significant defects sometimes go undetected until discovered during machining of the casting. In the event that the casting is defective and must be scrapped, time and effort devoted to machining is wasted. On the other hand, the distortion sometimes results in a false positive, i.e., identifying, as a defect, a feature that is not a defect. At the very least, false positives require an investigation, which can be time consuming. False positives can, even after investigation, result in a satisfactory casting being rejected as though it is defective one, thereby impacting production costs.
Various methods currently exist to help evaluate an x-ray image, e.g., contrast stretching, shift subtraction, sharpening, and/or filtering. These methods have been useful in radiographic inspection systems for objects that do not have large grains, for example fine-grained airfoils and microelectronics. None of these methods are effective for detecting relatively subtle but significant defects based on an x-ray image having a high level of distortion from large grains.